1. Field of the Invention
The present invention relates to a mark position detecting method and device for an aligner, which align between a mask and a semiconductor wafer and expose patterns on the mask onto the wafer, the method and device detecting a position of a rectangular mark formed on the wafer or the mask, and an aligner having such a mark position detecting device.
2. Description of the Prior Art
FIG. 8 shows a portion of conventional proximity aligner. The aligner is designed to align an alignment mark 13 on a mask 12 relatively to an alignment mark 11 on a semiconductor wafer 10. The wafer 10 is placed on a wafer stage 14, while the mask 12 is, for example, electrostatically absorbed to a mask stage 16 through a mask ring 15 fixed to the peripheral portion of the mask 12. A metallurgical microscope barrel 18 is mounted on a base plate of the mask stage 16 through a microscope stage 17. A light emitting from a white light source 19 passes along an optical fiber 20, through a half-mirror 21 and an objective lens 23 which are disposed inside the metallurgical microscope barrel 18, and then irradiates the wafer 10 and the mask 12. A reflected light from either the wafer 10 or the mask 12 passes through the objective lens 23, is reflected by the half-mirror 21, and is then formed into an image on a CCD in a two-dimensional camera 25 through a relay lens 24.
A video signal and synchronizing signals, which are output from the two-dimensional camera 25, are supplied to an image input unit 26. The image input unit 26 converts the video signal into a digital signal as a luminance data and generates an address on the basis of the synchronizing signal so as to store the luminance data at that address in an image memory 27. An image processing unit 28 detects a relative position of the alignment mark 11 with respect to the alignment mark 13 by processing the image stored in the image memory 27, and supplies the detected relative position to a controller 29. The controller 29 controls moving of either the wafer stage 14 or the mask stage 16 such that the relative position becomes a preset value, for example, 0. The alignment mark 11 can accurately be aligned to the alignment mark 13 by repeating the above-described aligning process a few times.
According to the standard, it takes 1/30 second for the two-dimensional camera 25 to output image data of one frame. Therefore, if the above-described aligning process is repeated, for example, three times, the image taking-in operation alone requires 0.1 second, thus reducing the throughput of the exposure process.
On a two-dimensional image 27a, shown in FIG. 9, in the image memory 27, a window W is set at the central portion thereof. In this window W, luminance data are accumulated in a direction indicated by Y in FIG. 9A for the purpose of averaging out the image signals, and an X-axis projected luminance distribution shown in FIG. 9B is obtained. Positions X1 and X2 of the negative peaks corresponding to the two edges of the alignment mark 11 are obtained, and the mean value thereof, X0=(X1+X2)/2, is obtained as the X-directional representative position of an alignment mark 11. In this way, the representative position X0 of the alignment mark 11 having a width from several to several tens of .mu.m can be detected accurately.
However, in the above-described position detection method, it takes a long time for accumulation in the Y direction, reducing the throughput of the exposure process. The above-described problem also arises in a projection aligner.